Capacitive force sensing devices suffer from several constraints which have limited their manufacturability and usefulness in real life applications. These constraints are known respectively as relaxation or creep, hysteresis, set, and off-axis loading.
Hysteresis is another limitation inherent to the use of various springs. When there is a difference in spring deflection at the same applied load—during loading and unloading—the spring is said to have Hysteresis. Hysteresis could come about from set, creep, relaxation and friction. Hysteresis will have the effect of limiting the usefulness of the capacitive force sensing device. Specifically, the spring must consistently and repeatedly return to its original position as the load is applied or removed. Failure to do so will cause erroneous readings.
Off-axis loading occurs when the direction of the applied load is not along the initial axis of the sensor. Off-axis loading can cause the capacitive plates to become non-parallel and significantly impact the measured capacitance and hence the load. Referring to FIG. 1, FIG. 1a illustrates an example of off-axis loading. Force 110 is applied to platform 120 and the force then gets transmitted to the compression spring 130. Since force 110 is along the initial axis of the sensor, the two capacitor plates 120 and 140 remain parallel. Referring to FIG. 1b, force 150 is applied in a manner, not along the original axis of the sensors 160 and 180, and not along the original axis of the compression spring 170. Consequently, plate 160 rotates to be perpendicular to the direction of force 150 and is no longer parallel to plate 180.
Many traditional springs such as helical springs or elastomeric springs (made from polymers, i.e. rubber or plastic) tend to suffer from all of the above constraints and consequently require special attention and design changes for building consistently accurate sensors.